A flexible resin boot of the type has a large port at one end to be fitted to the housing of a constant velocity joint of an automobile, and a small port at the other to be fitted to the axle thereof, and has a tapered bellows between the two ports. Applied to a constant velocity joint, it prevents grease from leaking out of the joint and prevents dust from entering the joint.
For forming such flexible boots, heretofore, chloroprene rubber has been used generally. However, flexible boots formed from chloroprene rubber are much expanded and deformed by the rotational centrifugal force, especially when they are rotated at high speed. In case where they are kept expanded and deformed under the condition for a long period of time, or where they undergo repeated expansion and contraction, they will be soon mechanically degraded and broken. Accordingly, the problem with such flexible boots is that their life is short.
Recently, thermoplastic elastomer resins such as thermoplastic polyester elastomer resins and the like having high elasticity have come to be used for forming flexible boots. The materials have the advantages of good heat resistance, good flexure resistance and high strength. However, flexible boots made from such high-elasticity thermoplastic elastomer resins are still problematic. Specifically, when the flexible boot is applied to a constant velocity joint in an automobile and when rotated therein while being bent at a wide angle, the mountains of its bellows are much rubbed against each other to make a noise and they are often worn away. In particular, in case where water adheres to the outer surface of the flexible boot, the noise is serious.
To solve the noise problem, proposed is adding silicone oil or fatty acid amide as a noise-preventing agent to thermoplastic polyester elastomer resins for flexible boots. For example, Japanese Patent Laid-Open No. 177971/1997 discloses a technique of adding a fatty acid amide to the resins for forming flexible boots.
When applied to the constant velocity joint in an automobile and when continuously rotated therein while being bent at a wide angle, the flexible resin boot of Japanese Patent Laid-Open No. 177971/1997 in which a fatty acid amide is added to a thermoplastic polyester elastomer resin makes no noise in early stages, but its noise-preventing effect could not last long. In fact, in the driving test of the flexible resin boot mounted constant velocity joint in an automobile, it has been found that the boot soon makes a noise after a certain period of time. To enhance and prolong the effect of the boot for noise prevention, the amount of the fatty acid amide to be added to the resin may be taken into consideration. However, increasing the amount of the fatty acid amide added to the resin results in the increase in the powdery fatty acid amide bled on the surface of the flexible boot, and the bled acid amide easily peels off. As a result, adding such an increased amount of the fatty acid amide is ineffective for actually enhancing and prolonging the effect of the boot for noise prevention. In addition, since the amount of the bled acid amide increases, the frictional factor between the large port or the small port of the boot and the housing of the constant velocity joint or the axle of an automobile lowers, and, as a result, the large port or the small port will slip more easily, whereby the boot will be dislocated to cause grease leakage from the joint. In fact, we, the present inventors have experienced grease leakage from the joint that worsens the ability of the boot to seal the joint.
The object of the present invention is to solve the problems as above, and to provide a flexible resin boot having the advantages of long-lasting noise prevention as well as a noise-prevention effect in early stages, sealability and durability.